1. Field of the Invention
The present invention relates to a machining apparatus for forming a plurality of dimples on a surface of a workpiece, in particular a machining apparatus for rapidly forming a large number of dimples, in an irregular pattern, on a surface of a mold for molding a light guide plate.
2. Description of the Related Art
In many cases, a light guide pipe made of translucent resin or the like, provided with a side light or a back light, is used as an illumination device for a liquid crystal display (LCD) used in a personal computer or a mobile-phone. As the LCD is required to have a uniform surface brightness, a large number of minimal protrusions (or dots), each having a hemispheroidal shape or a prism shape, are formed on the surface of the light guide pipe so as to emit scattered light from the light guide pipe. To this end, a large number of cavities (or dimples) are formed on a mold for molding the light guide pipe, by using a machining apparatus provided with a tool a tip of which has a pyramid shape or a cone shape. Such an apparatus is described in Japanese Unexamined Patent Publication No. 2005-67065, for example.
In general, when the dots are regularly arranged on the light guide pipe, a moire image may be generated on the display of the LCD due to interference with a color filter or the like. Therefore, the dots should be irregularly arranged and the dimples of the mold, corresponding to the dots of the light guide pipe, should also be irregularly arranged on the mold. In a machining apparatus of the prior art, the position or the coordinate of each dimple on the mold is appointed, one by one, in a machining program. In addition, Japanese Patent Publication No. 3402138 discloses a technique for irregularly arranging a plurality of dots, in which the coordinate of each dot is determined by using a mathematical function satisfying a certain constraint condition.
In general, the length of the diameter or one side of each dimple is approximately 10 μm. In some cases, the number of such dimples formed on the mold or the workpiece can amount to several tens or hundreds of thousands. In order to appoint all coordinates of such a large number of dimples in the machining program, the program must include tens or hundreds of thousands of lines even if only one line is required in the program for machining one dimple. On the other hand, when a particular mathematical function as shown in Japanese Patent Publication No. 3402138 is used, the coordinate of each dimple may be effectively determined. However, in view of the movement and the positioning of the tool or the workpiece, it takes time to form each dimple, because it is necessary to precisely form each dimple at the determined coordinate. When tens or hundreds of thousands of dimples must be formed, one day or more is necessary, even if only one second is required to form one dimple. Therefore, this technique is unsuitable for high-speed machining.